An optical waveguide device that uses an electro-optic crystal such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO2), is formed by forming a metal film of titanium (Ti) or the like on a part of a crystal substrate, to be thermally defused, or to be patterned, after which it is proton exchanged or the like in benzoic acid, to form an optical waveguide, and thereafter an electrode is provided in the vicinity of the optical waveguide. As such an optical waveguide device that uses an electro-optic crystal, there is known for example an optical modulator as illustrated in FIG. 1.
In FIG. 1, an optical waveguide formed on a substrate 100 comprises; an input waveguide 121, an optical branching section 122, a pair of branching waveguides 123 and 124, an optical multiplexing section 125, and an output waveguide 126. A signal electrode 131 and an earth electrode 132 are provided on the pair of branching waveguides 123 and 124, to form a co-planar electrode. In the case where a Z-cut substrate is used, in order to use the refractive index variation due to the electric field in the Z direction, the signal electrode 131 and the earth electrode 132 are arranged directly above the optical waveguides. More specifically, the electrodes are patterned with the signal electrode 131 on the branching waveguide 123, and the earth electrode 132 on the branching waveguide 124. Here in order to prevent the light that is propagated through the branching waveguides 123 and 124 from being absorbed by the signal electrode 131 and the earth electrode 132, a buffer layer (not illustrated in the figure) is provided between the substrate 100, and the signal electrode 131 and the earth electrode 132. For the buffer layer, an oxide silicon (SiO2) or the like of 0.2 to 2 μm thickness is used.
In the case where such an optical modulator is driven at high speed, the output end T2 of the signal electrode 131 is connected to the earth electrode 132 via a resistance (not illustrated in the figure) to make a travelling wave electrode, and a microwave electric signal RF is applied from the input end T1 of the signal electrode 131. At this time, due to the electric field generated between the signal electrode 131 and the earth electrode 132, the refractive indices of the branching waveguides 123 and 124 respectively change as +na and −nb, so that the phase difference of the light propagated on the branching waveguides 123 and 124 changes. Therefore, a light Lin input to the input port Pin is intensity modulated by Mach-Zehnder (MZ) interferometer, and modulation light Lout is output from an output port Pout. By changing the cross-section shape of the signal electrode 131 to control the effective refractive index of the microwave electric signal RF, and by matching propagation speeds of the light and the microwave electric signal with each other, high speed optical response characteristics can be obtained.
Furthermore, due to the variety of recent optical modulation formats (for example multi-valued modulation formats, optical polarization division multiplexing formats, and the like), there are many cases where signals corresponding to a desired optical modulation format are generated, by combining a number of conventional optical modulators such as illustrated in FIG. 1 (for example, refer to Japanese Laid-open Patent Publication No. 2008-122786).
In the above described configuration where a plurality of optical modulators are combined, in order to reduce the size of the overall optical modulator, it is effective to integrate respective optical modulators on a single chip (substrate). In the following description, individual optical modulators integrated on a single chip is referred to as “an optical modulation section”.
More specifically, the optical modulator illustrated in FIG. 2 is a configuration example of where two optical modulation sections 120A and 120B are arranged in parallel on a single substrate 100. The optical modulation sections 120A and 120B, similarly to the configuration illustrated in FIG. 1, each have an MZ type optical wave guide, a signal electrode, and an earth electrode. Furthermore, one light input end of an input optical branching section 111 that uses a 2×2 optical coupler is connected to an input port Pin positioned on one end face of the substrate 100, and the two light output ends of the input optical branching section 111 are each connected to input wave guides 121A and 121B of the respective optical modulation sections 120A and 120B. As a result, input light Lin from the input port Pin is bifurcated and guided to the respective optical modulation sections 120A and 120B. Moreover, the two light input ends of an output optical branching section 112 that uses a 2×2 optical coupler are each connected to output wave guides 126A and 126B of the respective optical modulation sections 120A and 120B, and one light output end of the output optical branching section 112 is connected to an output port Pout located on the other end face of the substrate 100. As a result, the modulation light output from the respective optical modulation sections 120A and 120B is multiplexed into one, and output to the outside from the output port Pout.
In the above described configuration, in the case of applying electric signals RFA and RFB from the outside to signal electrodes 131A and 131B of the respective optical modulation sections 120A and 120B, electrode input terminals are provided in a package (not illustrated in the figure) for accommodating the substrate 100. If electrode input terminals respectively corresponding to the optical modulation sections 120A and 120B are placed side by side on the side face on one side of the package, mounting of the substrate 100 can be facilitated, and the mounting footprint made small. In this case, for the respective signal electrodes 131A and 131B on the substrate 100, electrode pads formed near each of input ends T1A and T1B are arranged side by side on one side (the lower side in the figure) of the opposite side faces of the substrate 100. In the electrode pads, in order to connect to the outside (the electrode input terminals of the package) with wire bonding or the like, it is necessary to have a certain amount of spacing.
In the case where, as described above, the input ends T1A and T1B of the respective signal electrodes 131A and 131B are arranged with a predetermined spacing on one surface of the substrate 100, if desired to match the timing at which the light and electric signal interact in the respective optical modulation sections 120A and 120B, then as illustrated in the example of FIG. 2, it is necessary to bend the input leader line portion from the input end T1B of one signal electrode 131B up to on the branching waveguide 123B, to delay the electric signal RFB. More specifically, when the electric signals RFA and RFB are applied simultaneously to the input ends T1A and T1B of the respective signal electrodes 131A and 131B, then in order to synchronize these with the modulation lights output from the respective optical modulation sections 120A and 120B, if the point where the respective modulation lights are multiplexed in the output optical branching section 112 is PO, and the points on the respective branching waveguides 123A and 123B where the optical path lengths from the point PO become equal are PA and PB, it is necessary to make the electrical length of the portion from the input end T1B of the signal electrode 131B up to the point PB equal to the electrical length of the portion from the input end T1A of the signal electrode 131A up to the point PA.
However, in the configuration of FIG. 2, the propagation loss with respect to electric signals of high frequency becomes large on the side of the signal electrode 131B where the input leader line portion is curved. Therefore the modulation band width of the optical modulation section 120B becomes narrower than the modulation band width of the optical modulation sections 120A. When in order to widen the modulation band width of the optical modulation section 120B, the length of the portion where the light and electric field interact with each other (hereunder called the interaction portion) on the optical modulation section 120B side is made short, the drive voltage is increased. Therefore a problem arises in that as well as a high output driver amplifier being necessary, the power consumption of the optical modulator is increased.
Furthermore, in the configuration of FIG. 2, in order detour the input leader line portion of the signal electrode 131B on the substrate 100, it is necessary to widen the width of the substrate 100. Here the width of the substrate 100 is the length of the substrate 100 in the perpendicular direction (the y direction in the figure) with respect to the propagation direction (the x direction in the figure) of the light in the interaction portion of the respective optical modulation sections 120A and 120B. When in order to reduce the delay amount in the signal electrode 131B and to narrow the width of the substrate 100, the spacing of the input leader line portions of the respective signal electrodes 131A and 131B is made narrow, cross talk of the electric signals between the respective signal electrodes 131A and 31B is likely to occur. Therefore the noise component of the signal light output from the output port Pout increases. Furthermore, the earth electrode (common to both 132A and 132B) located between the input leader line portions of the respective signal electrodes 131A and 131B becomes narrow. Therefore the high frequency response characteristic of the signal electrodes 131A and 131B also deteriorates.